1. Field of the Invention
The present invention relates to a push button switch and a key switch. More particularly, the present invention relates to a push button switch of the type that an operation member for displacing a movable contact is provided movable in a depression direction and in a recovery direction opposite to the depression direction respectively of the key top, within a predetermined stroke defined by abutment of the operation member with a stopper, and the operation member is biased in the recovery direction by an elastic sheet member. Furthermore, the present invention relates to a key switch of the type that a key top is elastically biased by an elastic sheet member in a recovery direction opposite to a depression direction of the key top, and a torsion bar is provided to prevent trapping of the key switch by another component.
2. Related Background Art
There is shown in FIG. 1 the structure of a first example of a conventional push button switch of this type used, for example, as a key switch of a keyboard of an electronic desk-top calculator.
This switch uses a key sheet 1 made of a high elasticity material such as silicon rubber and having a hollow dome portion 1a. A movable contact ld is provided at the top inner surface of the dome portion 1a. As an operator depresses the key top 5 with a finger, a key stem 2 movably supported up and down by a frame 4 lowers to push down the dome portion 1a. The dome portion 1a is then elastically deformed so that the movable contact 1d lowers and contacts fixed contacts 3a and 3b on a printed circuit board 3 to short-circuit them, thereby entering a key input.
Thereafter, the key stem 2 reaches the bottom dead point of the vertical motion stroke, and abuts a base portion le of the key sheet 1 serving as a stopper at the bottom dead point. Impact at this abutment is absorbed by elastic deformation of the base portion 1e.
When the operator detaches the finger from the key top 5, the key stem 2 moves upward from the bottom dead point by elastic deformation energy stored in the dome portion 1a. If the flange 2a of the key stem 2 abuts directly with a top dead point stopper portion 4c of the frame 4, an impact sound of discordant noises is generated.
In view of this, as shown in FIG. 1, an elastic absorber such as an O-ring 6 is interposed between the flange 2a of the key stem 2 and the stopper, portion 4c of the frame 4, to thereby cushion the impact of the returning key stem 2 and absorb the impact sound.
With the above structure, however, an absorber such as an O-ring 6 is required to be mounted for each key switch. In addition, the number of absorbers must be the same as the number of key switches of a keyboard. Thus ,there arises the problem that the number of components and the number of assembly steps increase, resulting in high cost.
For an electronic apparatus having a keyboard, such as an electronic calculator, typewriter, and personal computer, a key switch frequently used has been configured as having a narrow, and long key top, to improve the operability of the device. In such a case there has been adopted a structure in which a torsion bar is provided to prevent a key switch having an elongated key top from being inclined and trapped by another component when it is depressed at one end portion.
The structure widely used heretofore for a key switch of a keyboard of a compact electronic apparatus such as an electronic calculator has an arrangement that an elastic sheet member called a key sheet mounted with a movable contact is used to elastically bias the key top in the recovery direction opposite to the depression direction.
The structure of a second example of a conventional key switch having a torsion bar and a key sheet will be described with reference to Figs. 2 and 3.
First, the basic structure of the key switch excepting the torsion bar mechanism will be described. Referring to FIG. 2 reference numeral 11 represents a key top which is formed in a narrow and long shape in the right and left direction as viewed in FIG. 2. A key stem 12 is fixedly connected to the lower middle surface of the key top 11.
The key stem 12 is fitted in a tubular guide portion 13c to be able to slide therein, the tubular guide portion being formed in a frame 13 of an electronic apparatus on which the key switch is assembled. The key stem 12 along with the key top 11 is able to slide in the up and down directions as viewed in FIG. 2 (in the recovery direction and the depression direction of the key top).
A key sheet 14 is provided below the key stem 12. The key sheet 14 is made of an elastic material such as silicon rubber and has a hollow dome (projected portion) 14b projected from a flat base portion 14a of the key sheet 14. A movable contact 14c made of conductive rubber is attached to the inner top surface of the dome portion 14b. At the top of the dome portion 14b, there is formed a ring portion 14d which abuts with the lower surface of the key stem 12 so that the dome portion 14b pushes the key stem 12 upward.
The key sheet 14 is tightly attached to a printed circuit board 15 at its base portion the printed circuit board 15, there are formed fixed contacts 15a and 15a constituting an input circuit of the key switch. The key sheet 14 is positioned such that the fixed contacts 15a and 15a face the movable contact 14c.
With the structure described above, when an operator depresses the key top 11 with a finger, the key top 11 along with the key stem 12 lowers to elastically deform the dome portion 14b, so that the movable contact 14c lowers to contact the fixed contacts 15a and 15a to short-circuit them and enter a key input. In this state, when the operator detaches the finger from the key top 11, the dome portion 14b takes the original shape to push up the key stem 12 which along with the key top 11 returns to the initial position before the key operation.
Next, the mechanism of the torsion bar will be described.
As shown in FIG. 2, a pair of torsion bar holders (hereinafter abbreviated as bar holders) 16 and 16 is mounted on the lower surface of the key top 11 at opposite end portions in the lateral direction, by press-fitting or adhering the upper ends thereof in or to the key top 11. Each bar holder 16 is inserted into a hole 13d of the frame 13. As shown in FIG. 3, a U-character shaped groove 16b is formed in the lower end portion of the bar holder 16 facing the base portion 14a of the key sheet 14. A torsion bar 17 is coupled movable to the key top 11 via the bar holders 16 and 16.
The torsion bar 17 is made of a metal rod and formed in generally a U-character shape. Opposite end portions 17c of the torsion bar 17 are bent perpendicular to an intermediate portion 17b both in the same direction. The tips of the opposite end portions 17c are bent perpendicular to the opposite end portions 17c and face each other, the bent portions of the tips serving as operation ends 17a. The intermediate portion of the torsion bar 17 is parallel to the lateral direction of the key top 11. As shown in FIG. 11, the intermediate portion 17b is fitted in an inverted U-character shaped groove 13b formed at the lower end portion of a rib 13a extending downward from the frame 13. The torsion bar 17 is rotatably squeezed and borne by means of the rib 13a and a holding member 18 provided on the base portion 14a of the key sheet 14. Namely, the torsion bar 17 is rotatably borne using as a fulcrum the intermediate portion 17b. The operation ends 17a of the torsion bar 17 are fitted in the grooves 17a of the holders 16 rotatable and movable in the lateral direction.
Next, the operation of the torsion bar mechanism will be described.
As an operator depresses the key top 11 at the left end portion thereof as viewed in FIG. 2 in the direction indicated by an arrow B, an angular moment is applied to the key stem 12 in the counter-clockwise direction as viewed in FIG. 2. Thus, the key top 11 along with the key stem 12 tends to incline within the angle range defined by the clearance between the key stem 12 and a guide portion 13c of the frame 13.
In this case, the bar holder 16 at the left end along with the key top 11 lowers to push down the left operation end 17a. As a result, the left end portion 17c rotates in the counter-clockwise direction as viewed in FIG. 3 using as a fulcrum the intermediate portion 17b.
Since the torsion bar 17 is made of a metal rod having a high rigidity, the right end portion 17c also rotates similar to the left although the torsion bar is subjected to torsional deformation more or less Therefore, the right operation end 17a pushes down the right bar holder 16 so that the right end portion of the key top 11 which is not depressed by the operator is also lowered. In this manner, the key top 11 is smoothly lowered while being inclined by a small amount.
According to the second example of a conventional key switch, in order to allow the operation ends 17a of the torsion bar 17 to smoothly move within the grooves 16b of the bar holders 16, the width of each groove 16b is set to have a suitable play (margin) while considering the dispersions and work tolerances of the diameter of the torsion bar 17. Similarly, the dimensions of the groove 13b of the rib 13a and the holding member 18 for bearing the also set to have a suitable play (margin).
However, with such a play, discordant click sounds will be generated when the key top 11 is depressed because the operation ends 17a and intermediate portion 17b of the torsion bar 17 abut with the inner surfaces of the grooves 16b and 13b.
Furthermore, the operation of the torsion bar mechanism delays from the operation of the key top 11 due to the presence of the play. If there is a large play, the inclined angle of the key top 11, when it is depressed at one end portion, will become so large that the key stem 12 is trapped by the guide portion 13c of the frame 13, before the operation of the torsion bar mechanism becomes effective.